1. Field of the Invention
The invention relates to fasteners for securing objects together. More specifically, a torque limiting fastener is presented to regulate the amount of torque that may be applied to such fasteners.
2. Discussion of the Related Art
It may be desirable to limit the amount of torque that can be applied to a screw-type fastener by a mounting tool when securing objects together, particularly in a hardware setting. Excessive torque can damage the integrity of the threads of the fastener by causing the threads to shear off inside of a threaded boreor it can damage the objects if they have inferior strength or hardness as compared to the fastener. Another common problem is the stripping of the tool interface on the head of the fastener, e.g., the slot into which a flat-head screwdriver fits, hampering the ability of the tool to turn the fastener. The problems of excessive torque, though, must be balanced against the need for sufficient tightening of the fastener to rigidly couple the fastener with the objects to be mounted.
Two common methods for limiting the torque borne by a fastener include: (a) using a torque-limiting tool to secure the fastener to the mounted objects; and (b) using a fastener designed with torque-limiting features. While torque-limiting tools are reliable, they also are expensive compared to simple mounting tools, such as standard screwdrivers, and may not be configured to work with fasteners having varying head geometries. Fasteners that have torque-limiting features have been designed to work with at least some standard mounting tools, such as socket wrenches. Examples of these types of fasteners are shown in U.S. Pat. No. 5,256,015, issued to St. Clair, and U.S. Pat. No. 5,713,705, issued to Grxc3xcnbichler. However, many of these types of fasteners are for one-time use and have component parts that break-off or otherwise fail when a predetermined amount of torque is applied, thus making it difficult, if not impossible, to remove the fastener once installed.
A screw-type fastener is provided having features for limiting the amount of torque that can be applied to the fastener to a predetermined value. The fastener is also configured to allow for easy removal from mounting with objects so that the fastener can be reused. Standard mounting tools may be used to install and remove the fastener from such mounting.
The torque limiting fastener comprises a fastener body with a threaded shaft, a head, and a bore formed in the head. The fastener further comprises a head insert sized to fit within the bore. The head insert has a central tool interface region configured to accept a variety of mounting tools for the application of torque to the insert.
A first set of protrusions are formed on an exterior portion of the sidewall of the bore and a second set of protrusions are formed on a perimeter wall of the head insert and are mateable with the first set of protusions. Upon the application of a torque to the head insert below a certain pre-determined value, the first set of protusions transfers this torque to the second set of protusions and rotate the fastener body, and consequently, rotate the shaft into a threaded bore for tightening. Once the applied torque exceeds a torque-limiting value, one of the first and second set of protrusions elastically deform and fail to maintain engagement. Continued over-torquing will not cause rotation of the fastener body in the tightening direction, thus protecting the structural integrity of the fastener and the mounted objects.
The torque-limiting features of the first and second sets of protrusions are realized by configuring one of the sets of protrusions to deflect and slip over the other set of protrusions at the torque limiting value. In one aspect, the first set of protrusions on the fastener body take the form of nubs and the second set of protrusions on the fastener head insert take the form of vanes that are configured for deflection. In another aspect, the position of the nubs and deflecting vanes are reversed such that the nubs are on the fastener head insert and the deflecting vanes are on the fastener body.
To allow for removal of the fastener body from mounting engagement with the objects, the fastener head insert must transfer torque and resulting rotation to the fastener body in a loosening direction opposite of tightening direction. If the fastener body is tightened with a torque that equals or exceeds the limiting value, the first and second set of protrusions will likewise fail to rotationally engage in the loosening direction. Therefore, a wedge and a corresponding wedge-shape recess are implemented to provide a mating surface that can transfer sufficient torque to the fastener body to unscrew the body from the mounting position within the objects. In one aspect, the fastener head insert has the wedge formed on a bottom surface of the insert and the fastener head has the wedge-shaped recesses formed into a recessed base surface of the head. In another aspect, the fastener head insert has the wedge-shaped recess formed into the insert bottom surface and the fastener head has the wedge formed on the recessed base surface.